Method of natural gas pressure reduction on the city gate stations

ABSTRACT

A method of natural gas pressure reduction at a City Gate station providing an elimination of the station&#39;s energy consumption for gas flow heating together with the creation of a cooling duty for further utilization which comprises connecting a vortex tube inlet with a gas flow line for applying the whole gas flow entering the City Gate, connecting a cold fraction gas flow outlet from the vortex tube with a heat exchanger for warming the cold fraction gas, and outletting the warmed cold fraction gas, connecting a vortex tube&#39;s hot fraction outlet with the warmed cold fraction outlet from the heat exchanger, to combine the hot fraction outlet with the warmed cold fraction outlet; and connecting the combined hot fraction and warmed cold fraction to the pipeline leaving the station, the method of the invention is also useful in connection with natural gas pressure reduction at a City Gate station equipped with a heater and a JT valve providing a reduction of energy consumption for gas flow heating, together with creation of a cooling duty for further utilization.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to gas pressure drop installations and, inparticular, to City Gate natural gas pressure reduction stations.

Description of the Prior Art

It is generally known and quite common in the industry that natural gashigh pipeline pressure has to be reduced at City Gate stations in orderto meet the low pressure gas distributing network requirements. Naturalgas is a mixture of hydrocarbon used for fuel.

This operation which is performed in so called JT valves cause (due tothe Joule-Thomson effect) a natural gas temperature drop. The greaterthe inlet/outlet gas pressure difference, the more of that drop value.As a result, the natural gas temperature after pressure reductionbecomes much less than the City Gate station location's groundtemperature. In most situations, the resulting temperatures are lessthan the critical temperature to freeze H₂ O.

Bearing in mind that both inlet and outlet City Gate pipelines aretypically underground, the freezing and ground distortion by heavingwill occur in the soil around the downstream pipeline, which leads tothe pipeline distraction and eventual failure. A potentially dangerouscondition.

To prevent such undesirable developments natural gas at the City Gatestations is constantly heated prior to the pressure reducing JT valve.The typical existing layout comprises a heater and a JT valve connectedin series, with the heater inlet connected with the gas pipelineentering the station and the JT valve outlet connected with the gaspipeline leaving the station. The quantity of energy required for theheating process depends on the pressure drop value, flow volume and theinlet gas temperature. This typically varies from station to station andcan vary within certain parameters within a single station depending onsystem feed in relation to downstream product demand.

Nevertheless, the total energy consumption for heating at any givenstation is always substantial due to the steady flow and the typicalvolumes of the gas involved.

A vortex tube design as set forth in U.S. Pat. No. 5,327,728 to Tunkelis particularly useful in connection with this invention.

SUMMARY OF THE INVENTION

To this end, the present invention consists in the provision of a methodof the natural gas pressure reduction at the City Gate station providinga substantial reduction of the station's energy consumption for heatingthe gas flow prior to pressure reduction, together with creation of thecooling duty for further utilization. The system harnessing this methodcomprises a vortex tube connected with the gas pipeline entering thestation prior to the existing heater with a vortex tube's cold fractionpipeline connected with a heat exchanger, and a vortex tube's hotfraction pipeline connected with the cold fraction pipeline leaving theheat exchanger and the pipeline containing both combined flows connectedwith the gas pipeline leaving the station's JT valve.

Another object of the present invention consists in the provision of amethod of the natural gas pressure reduction at the City Gate stationproviding a total elimination of the station's energy consumption forheating the gas flow together with creation of the cooling duty forfurther utilization. The system harnessing this method comprises avortex tube connected with the gas pipeline entering the station with avortex tube's cold fraction pipeline connected with a heat exchanger anda vortex tube's hot fraction pipeline connected with the cold fractionpipeline leaving the heat exchanger and the pipeline containing bothcombined flows connected with the gas pipeline leaving the station.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic flow diagram of an existing City Gate gastreatment system.

FIG. 2 is a schematic flow diagram of one embodiment of the presentinvention.

FIG. 3 is a schematic flow diagram of another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now more particularly to FIG. 1 which schematically shows anexisting conventional City Gate gas treatment system 10 and comprises anentry line 12, a heater 14 and a JT (Joule-Thomson) valve 16 and exitline 18.

A natural gas flow entering the station with the flow rate G_(o),pressure P_(o) and temperature T_(o) is heated in a heat exchanger andthen goes through a JT valve where the gas undergoes the pressure andthe temperature drop. As a result the natural gas leaves a station withthe required pressure P₁ and temperature T_(r) value. The temperatureT_(r) is set above of the ground water freezing temperature.

As is shown on the flow diagram of FIG. 2, which shows one embodiment ofthe invention, a station's gas treatment system 20 according to theinvention includes an existing heater 14 and JT valve 16 and also avortex tube 28 and a heat exchanger 32. A gas flow 22 enters the station20 with a flow rate G_(o), a pressure P_(o) and a temperature T_(o) andis divided into two parts to form two flow paths 24 and 26. One flowpath 24 G_(o) ¹ is directed through the existing system heater 14 and JTvalve 16. The other flow G_(o) ¹¹ is sent through path 26 (G_(o) ¹+G_(o) ¹¹ =G_(o)) to the Vortex Tube 28, where under the inlet/outletgas pressure ratio P_(o) /P₁ available, the gas undergoes an energy(temperature) separation and cold and hot fractions are created.

The vortex tube performance, in other words, the temperaturedifferences, as well as the actual cold and hot fraction flow rates, aredetermined in order to provide an actual cold fraction temperature T_(c)in output line 30 from Vortex Tube 28 lower than the temperature of thechosen medium to be cooled in the heat exchanger 32 and the hot fractionactual temperature T_(h) in output through line 34 higher than thetemperature T_(r) required for the gas leaving the station 20 throughline 36.

The cold gas or the cold fraction is then directed through line 30, thengoes through heat exchanger 32 where it is warmed to some determinedtemperature T_(c) ¹. After the heat exchanger 32, the warmed cold gas isdirected into line 38 and then mixes with the flow of hot gas from line34 and flows into line 40. These legs or lines 34 and 40 of the flowcircuit are contained within insulated pipes and valves.

It is important to emphasize that under a properly selected combinationof the vortex tube design and its mode of operation (at this point onemay use the disclosure of the U.S. Pat. No. 5,327,728) matched with theheat exchanger duty, the re-combined flow in line 40 (cold stream andhot stream) would have a final temperature, T₁ sufficiently warm enoughso that when mixed with the warmed gas flow after the JT valve, willproduce a final flow which is with equal to or above the requiredminimum gas temperature, (usually 35° F.) T_(r) sent downstream to theuser base through the underground pipeline system.

It should be understood that when T₁ is equal to T_(r) the station'senergy consumption should be reduced proportionally to the vortex tubeinput gas flow rate (G_(o) ¹¹), which no longer needs to be treated inthe heater; also, when T₁ is above T_(r) the heater's duty can bereduced additionally in order to meet the required value of T_(r) forthe final combined gas flow. In general, such a situation is typical inlate spring and summer when the heat exchanger using the ambient air asa heating medium provides a relatively high temperature of the coldfraction flow.

It also should be understood that the cooling duty provided in the heatexchanger by the Vortex Tube's cold stream can be utilized and put towork in a wide variety of applications. In the cooling duty availabilityestimations one has to be aware that both the natural gas temperaturedrops due to the vortex phenomenon and Joule-Thomson phenomenon (whichalso takes place in the vortex tube) are arithmetically added, so thatvery low actual gas temperatures may be achieved.

In evaluating the vortex tube performance in this flow diagram, oneneeds to take into account the variability of the flow rate on thestation which in general calls for variability of the vortex tube'scapacity. The most effective and reliable way to comply with thevariables is to have a set of the vortex tubes (two-three units of equalsize) with a total capacity of close or equal to the average stationflow rate.

This arrangement will provide a series (2 or 3) of vortex tubes to turnon and off separately as flow variabilities require. Such an approachprovides an opportunity to keep the vortex tube/tubes permanently loadedwith the station's basic flow rates. The JT valve is also permanentlyloaded with the balance (G_(o) -G_(o) ¹¹) of the station flow. Such aninstallation can be expected to accommodate any reasonable hourly/dailyload swings of the customer's gas demand.

It should be understood that the vortex tube's capacity depends on theactual inlet pressure value. In other words, the actual G_(o) ¹¹ flowrate varies according to the seasonably/daily/hourly pressure changes.

The actual number of vortex tubes on the station and their individualcapacity should be determined separately for each station--and thecapacity will also depend on the station's annual pattern of operation.Based on the selected total vortex tube/tubes capacity--among the otherappropriate input data, the proper size of the heat exchanger can bedetermined.

It should also be understood that with the appropriate combination ofthe operational conditions at the City Gate station's location, forexample with a relatively mild winter together with a relativelyconstant gas flow rate, an existing City Gate station's layout can bereplaced completely with the sole vortex tube's based system.

Referring now more particularly to FIG. 3 which shows anotherembodiment, a station's gas treatment system 40, the flow diagram isdifferent from the flow diagram described in FIG. 1 or FIG. 2. In thisembodiment, parts similar to the parts in FIGS. 1 and 2 have been raisedby 100.

As is shown on the flow diagram of FIG. 3, such a station's gastreatment system 50 according to the invention, includes a vortex tube128 and a heat exchanger 132. A gas flow 22 entering the station 50 witha flow rate G_(o), a pressure P_(o) and a temperature T_(o) is directedsolely to the vortex tube 128, where under the inlet/outlet gas pressureratio P_(o) /P₁ available, the gas undergoes an energy (temperature)separation and cold and hot fractions are created. The cold gas or thecold fraction then goes through line 130, heat exchanger 132 and afterbeing warmed to temperature T_(c) ¹, goes through line 138 to be mixedin line 136 with hot gas from line 134. The combined hot and cold flowwith the required pressure P₁ and temperature T_(r) leaves the stationdirectly through line 136.

While there has been shown and described what is considered to be thepreferred embodiments of the invention, various changes andmodifications may be made therein without departing from the scope ofthe invention.

What is claimed:
 1. A method of natural gas pressure reduction at a CityGate station equipped with a healer and a JT valve providing a reductionof energy consumption for gas flow heating, together with creation of acooling duty for further utilization comprising:connecting a vortex tubeinlet with a gas flow line prior to the gas flow's connection with aheater for applying a portion of the gas flow entering the City Gate;connecting a cold fraction gas flow outlet from the vortex tube with aheat exchanger for warming the cold fraction gas, and outletting thewarmed cold fraction gas; connecting a vortex tube's hot fraction outletwith the warmed cold fraction gas outlet from the heat exchanger, tocombine the hot fraction outlet with the warmed cold fraction outlet;and connecting the combined hot fraction and warmed cold fraction to thepipeline leaving the station's JT valve.
 2. The method of claim 1,including maintaining the temperature of the combined gas after leavingthe heat exchanger equal to the required minimum gas temperature afterthe JT valve.
 3. The method of claim 2, including the step ofproportionally reducing the station's energy consumption for gas flowheating relative to the vortex tube input gas flow rate.
 4. The methodof claim 1, including the step of proportionally reducing the station'senergy consumption for gas flow heating relative to the vortex tubeinput gas flow rate.
 5. The method of claim 1, including the step ofmaintaining the temperature of the combined gas after leaving the heatexchanger above the required minimum gas temperature after the JT valve.6. The method according to claim 5, including reducing the heatingimparted to the incoming gas line by the station's heater in order tokeep the final combined gas flow temperature equal to a given requiredvalue.
 7. The method according to claim 1, including reducing theheating imparted to the incoming gas line by the station's heater inorder to keep the final combined gas flow temperature equal to a givenrequired value.
 8. A method of natural gas pressure reduction at a CityGate station providing an elimination of the station's energyconsumption for gas flow heating together with creation of a duty forfurther utilization together with creation of a cooling duty for furtherutilization comprising:connecting a vortex tube inlet with a gas flowline for applying the whole gas flow entering the City Gate; connectinga cold fraction gas flow outlet from the vortex tube with a heatexchanger for warming the cold fraction gas, and outletting the warmedcold fraction gas; connecting a vortex tube's hot fraction outlet withthe warmed cold fraction gas outlet from the heat exchanger, to combinethe hot fraction outlet with the warmed cold fraction outlet; andconnecting the combined hot fraction and warmed cold fraction thepipeline leaving the station.
 9. The method of claim 8, wherein the gasflow inlet is connected solely with the vortex tube's inlet.
 10. Themethod of claim 8, wherein only the flows from the outlets of the vortextube are connected to the pipeline leaving the station.